Method for making paper-like and related non-woven products from acrylonitrile polymer fibers



Jan. 16, 1968 T c. SPENCE ETAL 3,364,102

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in an aqueous, non-drying 2O ml'nu/es and 56600003} Cu/ f0 s/ap/e Aer/91% [/n or a/read in fhar arm F/brU/a 749 ff) aqueous suspension woven web s/ruc fare lrreversfbfy dry 1% ca//a,oee ge/ ber' s/ruc fare lNvENToRs. 7/70 mas C. Spence. Frederick 6. 5a.; fwa 00 United States Patent 3,364,102 METHOD FOR MAKING PAPER-LEE AND RE- LATED NON-WOVEN PRUDUCTS FRGM AC- RYLGNTTRELE PULYMER FIBERS Thomas C. Spence and Frederick E. Eastwood, Yorktown,

Va, assiguors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed July 27, 1964, Ser. No. 385,287 11 Claims. (Cl. 162157) This invention relates to a method for improving the tear strength and fold endurance of paper and other related non-woven fibrous products manufactured from acrylonitrile polymer fibers. More particularly, the invention relates to preparing such improved fibrous products from wet-spun gel fibers of acrylonitrile polymer fibers.

The synthetic-hydrophobic textile fibers that were developed principally for textile end uses are now being employed in the development of numerous novelty and specialty papers and other non-woven products not traditionally associated with the textile industry. Particular emphasis has been placed on the preparation of paper and paper-like webs from these polymeric fibrous products either to be ultimately used in a typical paper end use or to be combined with other materials as in a laminate structure to improve the structural properties or its aesthetic characteristics. Fibers from acrylonitrile polymers have been found especially adaptable for the preparation of these non-woven fibrous products. For instance in US. 2,810,646, it is reported that paper having good properties can be prepared from water-fibrillated, wetspun filaments of acrylonitrile polymers of paper-making length. In US. 3,047,455, it is reported that the paper products of U.S. 2,810,646 can be made to have improved properties if the acrylonitrile polymer filaments are in a gel and uncollapsed condition while being beaten or fibrillated in an aqueous medium.

The conventional means for preparing a paper or paper-like web from the selected fibrous material is to subject an aqueous slurry or suspension of the fibrous material, generally cut to short staple or paper-making lengths, to a rather severe beating operation which causes the fibers to fibrillate so that a network of fine fibrils is made available to interlock and hold the sheeted-out web together. It is frequently noted, however, that the noncellulosic fibrous material, including acrylonitrile polymer fibers, for the most part are not readily or substantially fibrillatable with the ease and to the degree generally exhibited in the normally utilized cellulosic fibers or cellulosic materials such as wood pulp. Thus, beating the acrylonitrile polymer fibers to induce fibrillation frequently merely breaks the fibers into shorter lengths rather than producing fibrils along the fiber surface. It has now been observed that the flexibility of the fiber has an influence on its fibrillatability, that is, the more flexible the fiber the less the tendency for the fiber to break into smaller segments and the greater propensity for the fiber to fibrillate. Accordingly, the increased flexibility of the fiber that is to be utilized in the preparation of the web influences the fiber entanglement during sheet formation, which in turn affects the hydrodynamic behavior of dilute fibrous suspensions and wet-web strength. It also affects the compressibility and resilience of the paper during wet pressing. Moreover, an increase in fiber flexibility or pliability furthers the overall fibrillation of the pulp, which, as indicated, increases the bonding area in the sheet and thereby increases the density and strength of the final paper. The increased flexibility of the fiber will generally exhibit itself in the paper prodnot prepared from a slurry or suspension of these fibers in that the paper product will have significantly enhanced folding endurance or flexibility which is an especially important paper property. The stiffness or non-flexibility of a sheet is strongly influenced by the stiffness of the component fibers and by their degree of entanglement and of interfiber bonding. Fiber or fibril stiffness is approximately proportional to the fourth power of fiber diameter, and therefore it is of advantage to reduce the fiber or fibril diameter in order to effectuate flexibility and the general softness of the web product. Also, if the fibrils can be made to fibrillate into longer fibrils, greater entanglement of them generally results leading to higher interfiber bonding and hence, higher tear strengths.

It is the chief object and primary concern of this invention to provide a method for improving the properties, and particularly tear strength and fold endurance, of paper and related non-woven products prepared from aqueous suspensions or slurries of staple length acrylonitrile polymer gel fibers. It is a further object to provide a means for manufacturing a continuous web of acrylonitrile polymer fibers of excellent uniformity and utility with little or no modification of conventional paper-mak ing and related web-forming equipment.

The foregoing and additional objects and advantages are accomplished in and by practice of the present invention which comprises, in the method of manufacture of paper or paper-like and related non-woven products from acrylonitrile polymer gel filaments in an uncollapsed and never-dried condition, wherein the gel filaments are out into paper-making or staple lengths and are fibiillated by beating an aqueous suspension of the fibers, and then the fibrillated fibers are sheeted out to a web-like structure of predetermined dimensions, the improvement comprising priorto the fibrillation of the acrylonitrile polymer gel filaments in the aqueous suspension, subjecting the gel filaments to a mechanical crushing action so as to at least partially crush damage the gel filaments inducing longitudinal splitting therein, then subjecting the gel filaments while they are in a completely relaxed, unrestrained, free to shrink condition to a heated, aqueous non-drying medium at a temperature of between about 180 and 300 F. for a minimum period of time of between at least about 20 minutes and 5 seconds, the minimum time varying inversely with the temperature of the heated medium, and then cooling the heated gel fibers to at least about F.

The present method is schematically portrayed in the attached drawing.

Paper and paper-like products and related non-woven structures prepared by the present invention have significantly increased tear strengths and additionally, particularly where a high degree of beating to induce fibrillation is given the gel fibers, the paper and like structures have significantly increased fold endurance without inducing any depreciation in tear strength, which is often associated with increases in fold endurance.

The crush damage that is induced in the gel fibers in accordance with the present invention is that damage which causes a longitudinal splitting of the fiber at the damage point. Descriptively, it can be likened to the type of splitting that is encountered when a bamboo rod is bent until it ruptures, the shattering or splitting traveling longitudinally of the rod at the bend point without any actual severing of the rod into two lengths. We have found by inducing such longitudinal splitting into the gel fibers before they are subjected to the conventional beating operation to fibrillate the fibers, that webs cast from the fibrillated mass have an excellent combination of web properties, notably tear resistance and fold endurance. Evidently, brils of much longer lengths and smaller diameters result, and breakage into short lengths is reduced.

Gel filaments either in the form of cut staple lengths or continuous filaments, e.g., a tow of gel filaments can be crush damaged to induce the longitudinal splitting. Ad-

vantageously and beneficially, the crush damaging is carried out on the gel filaments while they are in long con 'tinuous lengths and preferably on a bundle or tow of the filaments. The crush damage action can be supplied by any suitable means that will induce the requisite longitudinal splitting, including cooperating crush rollers or the like pressure regulated constrictions such as in serrated belt crimpers and meshing gears like those encountered in gear crimpers. Preferably, a stulfing box type crimper is employed wherein the tow of gel filaments is caused to fold back on itself while being forced against a resistance set to release at a predetermined pressure, for example, the type discussed in US. 2,917,784.

The actual force required to induce the crush damage, longitudinal splitting of the gel fiber or filament will vary depending on several factors which may include the fiber diameter or denier; the polymer composition, i.e., what and how much of different monomers are copolymerized with the acrylonitrile in the polymer; the spinning system including solvents and coagulants involved; the gel structure of the fiber, e.g., how much water or other inert liquid is in the structure; the degree of orientation or stretching given the fiber (preferably, the gel filaments are at least partially oriented before being crush damaged); and the temperature of the crush-damaging operation. Beneficially, the crush damaging is done at about 25-35 C., or less, temperatures above about 60 C. (140 F.) generally leading only to a flattening of the gel fiber without any longitudinal splitting. Generally, it can be said that the crush damaging of the fiber is significantly beyond that which would be normally acceptable limits it the fiber were to be designed as a textile fiber for conventional textile end uses such as weaving, knitting, etc. That is, the fibers, after crush damaging in accordance with the invention, would not run with any acceptance on conventional textile fiber handling and processing equipment. Thus, it is frequently observed that gel fibers and resulting irreversibly dried fibers therefrom, after being crush damaged as provided by the invention, will have fiber strengths of perhaps to Me less than gel fibers that are not crush damaged as provided herein. Advantageously, crush damaging so as to create a tensile strength 1055 of at least about /3 from the tensile strength of undamaged fibers is employed. Depending on the end purpose of the web, any notable degree of crush damaging to induce longitudinal splitting will be beneficial, and the artisan can choose the required pressures and number and severity of angles of crimps or folds to cause an advantageous longitudinal splitting, which will be evidenced in the improved properties'of the web formed therefrom as mentioned. It is not essential that the longitudinal splitting be continuous along the total length of the fiber. For that matter, this is not ordinarily preferred as some integralness as a fiber is desired 50 that a core or hub is available from which fibrils project. Often the splitting can be observed with the naked eye, depending on the denier, and certainly under a microscope.

' After the gel fibers'have been crush-damaged as desired, they are heated in an aqueous medium before being sub jected to the beating or fibrillating in aqueous suspension. The heating in the aqueous medium apparently adds toughness to the. gel fibers as well as stability. Gel fibers treated by this means are much more resistant to breaking up into short segments during the beating and thus the creation of fines is minimized, but more important, long fibrils are encouraged which adds to web flex strength, particularly when long beating cycles are used. Additionally, it has been found that without this heating treatment as prescribed, oftentimes the cast web when drying ruptures due to shrinking forces, and it becomes near impossible to manufacture a continuous web without substantial modification of the Web forming and drying equipment.

As indicated, the heating of the gel filaments is to be done under conditions such that the gel structure is not 7 collapsed or destroyed. To this end, preferably, the fibers are heated in a non-drying aqueous medium, and advantageously a water bath is employed. Temperatures from about 180 to about 300 F. can be used for this heating, and when a water medium is employed, in order to achieve the temperatures above the boiling point, a pressurized system may be employed, similar to pressure cooker. Saturated steam or the like can also be utilized, care being employed to assure that the gel fibers do not become dried and the gel structure does not become collapsed.

The time for the heating will depend primarily on the temperature of the heating medium and the time is important in order to provide the beneficial webs and Web properties of the end product. Thus, when heating the fibers at about 180 F., a residence time of about 20 minutes will .be necessary before any notable improvements will be evidenced. On the other hand, at 300 F. a residence time of about 5 seconds is oftentimes suflicient to provide notable improvement. Temperatures much below about 180 F. usually either require excessive heating residence times or do not notably eifect any particular beneficial results at all. Temperatures above about 300 F. usually do not materially add any incremental improvements in the web preparation or properties. Advantageously, residence times in the heating aqueous medium are at least about 3 minutes to about 10 seconds at temperatures, varying inversely with the time, of between about 210 and 300 F. For example, it is beneficial if a boiling water bath is used, e.g., about 212 F., to employ a residence time of about 5 minutes or more.

The gel fiber either in continuous or long lengths or in staple lengths can be treated in the heated aqueous medium. It is important in either case that the fibers be in a relaxed, unrestrained and completely free to shrink con dition during the heating period.

After the heating of the gel fibers in the aqueous medium it is an essential feature that the fibers be cooled to a temperature less than about F. prior to the heating operation or before any intermediate handling treatment that is apt to put a strain, stretching or elongating force on the gel fibers. If the gel fibers are in the form of continuous or long length filaments during the heating treatment which are to be cut to shorter lengths prior to the beating operation, then it is important that the fibers be cooled as indicated before they are so cut. Cutting usually imposes sufficient strain on the fibers so that whatever elfect the heating treatment may have imposed on the fibers is diminished by the strain or like force created during the cutting unless the fibers are first cooled to less than about 125 F. Likewise, the beating itself of the fibers in a hot condition tends to destroy the toughening imparted to the fibers during heating, and accordingly, the fibers are cooled prior to beating them. Cooling can be accomplished by air cooling (without permitting the gel fibers to become dried or collapsed) or by dunking or immersion in a cooled water bath or the like, Conveniently, the aqueous medium that is to be used as the fiber suspending medium can be used as the cooling medium. Of course, the final temperature of the suspension should be less than about 125 F. If extended beating times are to be employed, it may be desirable to provide the beating bath with a cooling jacket to keep the temperature down, as the beating energy is expended as heat and the suspension temperature may rise excessively.

After the paper web or like non-woven fibrous product is sheeted out from the beaten aqueous suspension, it is dried by any convenient means such as by passing it over heated rolls or through a hot air or radiant heated oven. 7

During the drying the gel fibers become irreversibly dried, destroying or collapsing the gel structure. Drying temperatures in the neighborhood of 125-140 C. are beneficially employed, although somewhat lower temperatures can be used with longer drying times. 1

scope of the invention in any way but merely as demonstrative of the invention.

Homopolymeric acrylonitrile fibers were wet-spun from an aqueous about 60 weight percent zinc chloride spinning solution into an aqueous about 43 weight per cent zinc chloride coagulation bath. The coagulated filaments were withdrawn from the coagulation bath, and washed substantially free of any residual zinc chloride, after which they were oriented by stretching the gel or aquagel filaments to about 13.5 times their extruded length in an aqueous bath at about 95100 C. Following the orientation of the aquagel fibers, they were collected while in the never-dried aquagel condition and separated into four portions. The first portion, a control sample, designated A, was cut into A-inch length staple. A second portion, designated B, was crimped at about 25 C. in a stuffing box crimper so as to have about to crimps per lineal inch of the filament whereby longitudinal fracturing in the gel filaments was induced, the crimped filaments having about /3 the tensile strength of the uncrimped filaments. The crimped filaments were cut into Ai-inch staple lengths. A third portion, designated C was cut to Ai-inch staple lengths, boiled in water for about 5 minutes and cooled to about C. (77 F.). A fourth portion, designated D, was crimped in the same manner as sample B, then boiled in a water bath (about 212 F.) for about five minutes, and cooled to about 25 C. and then cut to fii-inch staple lengths. Portions of each of samples A, B, C, and D were then separately suspended in water and the resultant slurries were beaten for 15, and 45 minute intervals in a Waring blendor set at low speed. Handsheets of the beaten material were formed using a Noble & Wood laboratory handsheet machine and then dried on a screenbacked contact drier for 2 /2 minutes at 126 C. Tear resistance of the handsheets was measured on an Elmendorf tearing tester and the folding endurance of the dried sheets was measured with an M.I.T. tester, following TAPPI standards. All the values obtained were standardized to an 8" X 8" handsheet (basis weight 45 pounds per 25 x 40"/50O ream) The results are set forth in the following tables.

Mll. DOUBLE FOLD CYCLES TO BREAK Beating Time, minutes Sample C t ol 2,070 3 420 1 330 ifii (lih l sh-damagein 1,100 2: 600 41010 (C) Boiled 500 2,205 6,300 (D) Crush-damaged and boiled 2, 020 6, 460 10, 900

ELEMENDORF TEAR, GRAMS PER 45 LB. BASIS SHEET Any of the commercially available beating or fibrillation equipment generally employed in the paper-making industry, and, the sheeting out and general paper formation equipment employed in the preparation of conventional paper may be utilized in the preparation of the paper or paper-like products from the suspension of fibrillated acrylonitrile gel filaments as described and discussed herein.

It is to be underst'ood that although the invention has been described and discussed generally with reference to paper and paper-like products, the invention is applicable to the preparation of any non-woven fibrous shape demanding the aqueous suspension of fibrillated gel filaments as the web or fibrous product forming source.

The present invention is applicable to preparing paperlike products from acrylonitrile polymer gel fibers which are fabricated from fiber-forming acrylonitrile polymers that contain in the polymer molecule at least about weight percent of polymerized acrylonitrile, including homopolymeric acrylonitrile, which are wet spun in and with systems that are adapted to utilize aqueous coagulating liquids in the spinning operation, such as systems wherein ethylene glycol, dimethylformamide, dimethylacetamide, dimethylsulfoxide, butyrolactone and the like or the various saline polyacrylonitrile'dissolving solvents are employed as spinning solution solvents for the polymer and are also present in non-polymer dissolving quantities in the aqueous coagulating liquid used in the spin bath. Gel fibers that are prepared by spinning a solution of the polymer in an organic solvent into air whereby some of the solvent is evaporated to solidify the fiber, i.e., dry spinning, and the remainder of the solvent in the fiber is interchanged with water while the freshly formed her is still in a swollen state can also be employed in the present invention.

The utile, known aqueous saline solvents for the various fiber forming acrylonitrile polymers and polyacrylonitrile for wet spinning processes include zinc chloride, the various thiocyanates such as calcium and sodium thiocyanate, lithium bromide, salt mixtures of the so-called lyotropic series, and others recognized by the art as has been disclosed, among other places, in United States Letters Patent Nos. 2,140,921; 2,425,192; 2,648,592; 2,648,- 593; 2,648,646; 2,648,648; 2,648,649; and 2,949,435. Advantageously, aqueous zinc chloride solutions are used for the purpose.

Exemplary of some of the monomeric materials that may be employed with the acrylonitrile in the preparation of the acrylonitrile polymer and copolyrner fiber forming systems and treated in accordance with the practice of the present invention include allyl alcohol, vinyl acetate, acrylamide, methacrylamide, methyl acrylate, vinyl pyridine ethylene sulfonic acid and its alkali metal salts, vinyl benzene sulfonic acid and its salts, 2-sulfoethylmethacrylate and its salts, vinyl lactams such as vinyl caprolactam and vinyl pyrrolidone, etc. and mixtures thereof.

As indicated, after acrylonitrile polymer fibers have been wet spun they are most frequently water washed or washed with an aqueous inert solution to remove any residual polymer solvent from the freshly formed filaments, thus forming an intermediate fiber product often referred to as a gel or aquagel filament. Thoroughly washed acrylonitrile polymer aquagel fibers, incidentally, are usually found to contain up to about 6 parts by weight of water (including residual extrinsic or exterior water associated therewith) for each part by Weight of dry polymer therein. More frequently, washed acrylonitrile aquagel polymer fibers are found to contain from about 3 to 4 parts by weight of water for each part .by weight of polymer. However, the actual amount of water or other inert liquid in the gel stnucture is not deemed critical except to the extent that the gel structure is maintained and is in an uncollapsed condition during the carrying out of the preparation of the paper-like and related nonwoven fibrous products in accordance With the invention.

What is claimed is: v

1. In the method of preparing paper-like and related non-woven fibrous webs from gel filaments of a polymerized ethylenically unsaturated monomeric material containing at least about 80 weight percent polymerized acrylonitrile wherein the gel filaments are cut to staple lengths and the cut lengths are then fibrillated by a heating action in an aqueous suspension, after which the fibrillated fibers are sheeted out into a web-like structure of pre-deterrnined dimensions followed by irreversibly drying of the web-like structure, the improvement comprising, prior to the fibrillation of the cut gel filaments in the aqueous suspension,

(a) subjecting the gel filaments to a mechanical crushing action so as to at least partially crush damage the gel filaments inducing longitudinal splitting there- (b) subjecting the crush-damaged gel filaments while they are in a completely relaxed, unrestrained, free to shrink condition to a heated, aqueous non-drying medium at a temperature of between about 180 and 300 F. for a minimum period of time of between at least about 20 minutes and 5 seconds, the minimum time varying inversely with the temperature of the aqueous medium; and,

(c) cooling the heated gel fibers to at least about 2. The method of claim 1, wherein said polymerized ethylenically unsaturated monomeric material is polyacrylonitrile.

3. The method of claim 1, wherein said aqueous medium is a water bath.

4. The method of claim 1, wherein said gel filaments are heated in said aqueous medium at a temperature of between about 210 and 300 F. for a minimum period of time of between at least about 3/minutes and seconds, the minimum time varying inversely with the temperature.

5. The method of claim 1, wherein said gel filaments are crush-damaged whereby longitudinal splitting is induced therein by crimping the gel filaments with a force and degree of crimp such as to induce a tensile strength loss in the crush-damaged filament of at least about /3 from the tensile strength of a like but uncrimped gel filament.

6. The method of preparing paper-like and related non-woven fibrous webs comprising (a) providing a plurality of continuous never dried,

at least partially oriented, gel filaments of a polymerized ethylenically unsaturated monomeric material containing at least about weight percent polymerized acrylonitrile;

(b) subjecting said .gel filaments to a mechanical cnushing action so as to at least partially crush damage the gel filaments inducing longitudinal splitting therein;

(c) cutting said crush-damaged gel filaments to staple lengths;

(d) subjecting said crush-damaged and cut gel filaments while they are in a relaxed, unrestrained, free to shrink condition to a heated, aqueous non-drying medium at a temperature of between about 180 and 300 F. for a minimum period of time of between at least about 20 minutes and 5 seconds, the

minimum time varying inversely with the temperature of the aqueous medium;

(e) cooling the heated, c-ut gel filaments to at least about F.;

(f) subjecting the cooled gel filaments in an aqueous suspension to a heating action causing the gel filaments to fibrillate; and

(g) sheeting out a web of the gel filaments from the aqueous suspension and irreversibly drying the web.

7. The method of claim 6, wherein said polymerized ethylenically unsaturated monomeric material is polyacrylonitrile.

8. The method of claim 6, wherein said aqueous medium is a water bath.

9. The method of claim 6, wherein said gel filaments are heated in said aqueous medium at a temperature of between about 210 and 300 F. for a minimum period of time of between at least about 3 minutes and 10 seconds, the minimum time varying inversely with the temperature.

10. The method of claim 6, wherein said gel filaments are crush-damaged whereby longitudinal splitting is induced therein by crimping the gel filaments with a force and degree of crimp such as to induce a tensile strength loss in the crush-damaged filament of at least about /3 from the tensile strength of a like but uncrimped gel filament.

11. A web prepared according to the method of claim 6.

References Cited UNITED STATES PATENTS 3/1963 Wishman 2 64--342 7/1962 Holmes et al 162-157 

1. IN THE METHOD OF PREPARING PAPER-LIKE AND RELATED NON-WOVEN FIBROUS WEBS FROM GEL FILAMENTS OF A POLYMERIZED ETHYLENICALLY UNSATURATED MONOMERIC MATERIAL CONTAINING AT LEAST ABOUT 80 WEIGHT PERCENT POLYMERIZED ACRYLONITRILE WHEREIN THE GEL FILAMENTS ARE CUT TO STAPLE LENGTHS AND THE CUT LENGTHS ARE THEN FIBRILLATED BY A BEATING ACTION IN AN AQUEOUS SUSPENSION, AFTER WHICH THE FIBRILLATED FIBERS ARE SHEETED OUT INTO A WEB-LIKE STRUCTURE OF PRE-DETERMINED DIMENSIONS FOLLOWED BY IRREVERSIBLY DRYING OF THE WEB-LIKE STRUCTURE, THE IMPROVEMENT COMPRISING, PRIOR TO THE FIBRILLATION OF THE CUT GEL FILAMENTS IN THE AQUEOUS SUSPENSION, (A) SUBJECTING THE GEL FILAMENTS TO A MECHANICAL CRUSHING ACTION SO AS TO AT LEAST PARTIALLY CRUSH DAMAGE THE GEL FILAMENTS INDUCING LONGITUDINAL SPLITTING THEREIN; (B) SUBEJCTING THE CRUSH-DAMAGED GEL FILAMENTS WHILE THEY ARE IN A COMPLETELY RELAXED, UNRESTRAINED, FREE TO SHRINK CONDITION TO A HEATED, AQUEOUS NON-DRYING MEDIUM AT A TEMPERATURE OF BETWEEN ABOUT 180* AND 300*F. FOR A MINIMUM PERIOD OF TIME OF BETWEEN AT LEAST ABOUT 20 MINUTES AND 5 SECONDS, THE MINIMUM TIME VARYING INVERSELY WITH THE TEMPERATURE OF THE AQUEOUS MEDIUM; AND, (C) COOLING THE HEATED GEL FIBRS TO AT LEAST ABOUT 125*F. 